News Release: NREL Develops Switchable Solar Window

November 27, 2017

Demonstration device dynamically responds to sunlight by transforming from transparent
to tinted while converting sunlight into electricity

Thermochromic windows capable of converting sunlight into electricity at a high efficiency
have been developed by scientists at the U.S. Department of Energy’s National Renewable
Energy Laboratory (NREL).

Relying on such advanced materials as perovskites and single-walled carbon nanotubes,
the new technology responds to heat by transforming from transparent to tinted. As
the window darkens, it generates electricity. The color change is driven by molecules
(methylamine) that are reversibly absorbed into the device. When solar energy heats
up the device, the molecules are driven out, and the device is darkened. When the
sun is not shining, the device is cooled back down, and the molecules re-absorb into
the window device, which then appears transparent. A video showing the device switch
can be seen here.

The NREL-developed demonstration device allows an average of 68 percent of light in
the visible portion of the solar spectrum to pass through when it’s in a transparent,
or bleached, state. When the window changes color—a process that took about 3 minutes
of illumination during testing—only 3 percent is allowed through the window. Existing
solar window technologies are static, which means they are designed to harness a fraction
of the sunlight without sacrificing too much visible light transmission needed for
viewing or the comfort of building occupants. “There is a fundamental tradeoff between
a good window and a good solar cell,” said Lance Wheeler, a scientist at NREL. “This
technology bypasses that. We have a good solar cell when there’s lots of sunshine
and we have a good window when there’s not.”

The proof-of-concept paper published in Nature Communications established a solar power conversion efficiency of 11.3 percent. “There are thermochromic
technologies out there but nothing that actually converts that energy into electricity,”
Wheeler said. He is the lead author of the paper, “Switchable Photovoltaic Windows
Enabled by Reversible Photothermal Complex Dissociation from Methylammonium Lead Iodide.”

His co-authors, all from NREL, are David Moore, Rachelle Ihly, Noah Stanton, Elisa
Miller, Robert Tenent, Jeffrey Blackburn, and Nathan Neale.

In testing under 1-sun illumination, the 1-square-centimeter demonstration device
cycled through repeated transparent-tinted cycles, but the performance declined over
the course of 20 cycles due to restructuring of the switchable layer. Ongoing research
is focused on improving cycle stability.

The path to commercialization of the technology was explored last year during a two-month
program called Energy I-Corps. Teams of researchers are paired with industry mentors to learn what customers want
of the technology and develop viable ways to reach the marketplace. Lance Wheeler
and Robert Tenent, the program lead for window technology at NREL and co-author on
the paper, teamed up to develop a market strategy for a product they called SwitchGlaze.
The effort was funded by the Emerging Technologies program within the Department of
Energy's Building Technologies Office.

Wheeler said the technology could be integrated into vehicles, buildings, and beyond.
The electricity generated by the solar cell window could charge batteries to power
smartphones or on-board electronics such as fans, rain sensors, and motors that would
open or close the windows as programmed.

The research was funded by Solar Photochemistry program within the Department of Energy’s
Office of Science along with contribution from NREL’s Laboratory Directed Research
& Development program.

NREL is the U.S. Department of Energy's primary national laboratory for renewable
energy and energy efficiency research and development. NREL is operated for the Energy
Department by The Alliance for Sustainable Energy, LLC.